With the advent of very low cost electronic integrated circuit technology, it has become practical to include a functional electronic tone generator device within and as an integral part part of a publication, such as a songbook containing songs suitable for play upon a keyboard of the tone generator device. Such tone generator devices have typically been very low cost, self contained, battery operated "throwaway" units. The typically have included a housing usually formed of a molded plastic material, a printed circuit card base carrying a power source such as a battery formed of a plurality of mercuric oxide cells, an integrated circuit tone generator, an audio tone transducer and a matrix of sets of typically interdigitated conductive traces forming a keyboard layout. The keyboard was completed in accordance with conventional membrane switch technology by providing a thin, continuous plastic film sandwich overlay for the keyboard layout, with conductive material disposed in the areas adjacently facing the sets of conductive traces. A spacing film between the surface carrying the conductive material and the printed circuit card had openings at each conductive area and functioned to hold the conductive area away from the underlying and oppositely facing traces. When the conductive film was depressed by the finger, the conductive material completed the contact between the interdigitated fingers of the traces of the set, and a tone was generated and reproduced aurally by the transducer. Thus, the tone generator could be used in the songbook in order to play the notes of the tunes printed therein.
One drawback of the prior art approach was that the thin film spacer could effectively provide for openings (active contact regions of the key) which were relatively small. Such small openings, and corresponding small areas of exposed conductive material required considerable dexterity on the part of the player to pick out the precise portion of each key which was active. With small children, such keyboards proved unreliable because the children were unable effectively to depress each key and thereupon became quickly frustrated with the songbook.
Another drawback of the prior art was the closeness of the spacing between each of the interdigitated fingers of each key. Such close spacing, together with typical very high input impedances of the low power (CMOS) electronic tone generator circuit, made it particularly susceptible to moisture. Humidity, or other moisture penetrating the keyboard, caused it to turn on and remain on, thereby depleting the power source and effectively ending the usefulness of the entire device.